A technique in which a transistor is formed using a semiconductor material has attracted attention. Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). Although silicon-based semiconductor materials have been widely used as semiconductor materials applicable to the transistor, oxide semiconductors have been attracting attention as alternative materials.
Patent Document 1 discloses an example in which a transistor including an oxide semiconductor in a channel formation region (hereinafter referred to as an oxide semiconductor (OS) transistor) is used in a dynamic random access memory (DRAM). The OS transistor has extremely low leakage current in an off state (off-state current); thus, a DRAM having a low refresh frequency and a low power consumption can be formed.
Patent Document 2 discloses a nonvolatile memory including an OS transistor. Unlike the flash memory, the nonvolatile memory has unlimited cycling capability, can easily operate at high speed, and consumes less power.